N/A
The placement of an optical filter in an optical fiber device to reflect a particular wavelength is well known. Single fibers and multi-fiber couplers all may include an optical filter that is of the fiber Bragg grating (FBG) type. A fiber Bragg grating includes a plurality of grating elements formed by intense beams of UV light that are transverse to the longitudinal axis of the optical fiber. The UV light induces a permanent change in the index of refraction in each grating region within the optical fiber.
Two examples of fiber optic devices incorporating a FBG are shown in FIGS. 1 and 2. FIG. 1 shows a fused/tapered add/drop multiplexer (ADM) 100. The ADM 100 includes an input fiber 102 in which at least one wavelength of light is input into the coupler. The FBG is written in the 100% coupling grating region 110. The wavelength selectively reflected by the FBG (the resonant frequency)is reflected and exits out of the drop fiber 104. An add fiber 106 may be used to couple a selected wavelength into the output fiber 108. The output fiber 108 will provide an output to the wavelengths input from the input fiber 102 and not reflected by the FBG and a wavelength coupled into the coupler from add fiber 106.
FIG. 2 illustrates a Mach-Zehnder Interferometer add/drop multiplexer (MZI-ADM) 200. The MZI-ADM 200 includes as inputs to the coupler input fiber 202 and add fiber 212, a first 50% coupler region 206, an interfering arm region 208 in which the FBG is written, a second 50% coupler 210, and as outputs a drop fiber 204 and an output fiber 214. Due to the interferometric nature of the device, the position of each component is extremely important. As such, there can be little or no strain placed on the first and second 50% coupler regions.
The particular wavelength that is reflected by the FBG is a function of the index of refraction of the optical fiber, the spacing of the grating elements and the induced change in the index of refraction of the grating elements. The index of refraction of an optical fiber, the grating elements, and the spacing of the grating elements are dependent upon the temperature of the grating region and the strain, if any, on the grating region. Therefore, once a FEG has been written into the grating region of an optical fiber, any change in the temperature or strain on the grating region may change the resonant frequency of the FBG.
One method for compensating an FBG for a change in temperature is to pre-strain the grating region of the optical fiber containing an FBG. In this way any increase or decrease in temperature may be compensated for by reducing or increasing the strain on the grating region containing the FBG respectively.
Although it is well known to pre-strain the region of the grating region of an optical fiber containing the FBG to compensate for a change in temperature, previous attempts have not proved adequate. Among the problems encountered with the previous attempts are that only single fibers could be compensated and that multiple fibers could not be compensated individually. Another problem besides being able to compensate only a single fiber is the inability to compensate a coupler such as a MZI-ADM due to the need to account for strain sensitivity of the couplers that need to remain stress free. Another problem with the previous attempts is that the FBG must be written prior to installation in the compensation apparatus. This is particularly a problem when compensating mutli-fiber couplers such as an ADM or a MZI-ADM because the FBG region can not be twisted or mishandled during installation. Since many of the previous attempts at temperature compensation have included load tensioning devices, such as screws and nuts that when applied to the fiber itself, increases the risk of twisting and damaging the FBG. Still another problem with the previous methods of temperature compensation is that in some methods the fiber region containing the FBG had to be in contact with the compensating member. Since the optical field in the waist or taper region of an add/drop multiplexer or Mach-Zehnder Interferometer is exposed, contact with another surface would cause an increase in the attenuation of the optical signal.
What is needed in the art therefore is a temperature compensation apparatus for a fiber region containing an FBG that is able to independently compensate multiple fibers, compensate a Mach-Zehnder Interferometer, compensate an add/drop multiplexer, does not apply torque or twist to the fiber region, does not require the optical field in the waist or taper region of a coupler to be in contact with any surface, and allows the FBG to be written to the selected fiber region after installation and assembly of the temperature compensation apparatus.
The present invention provides for a passive temperature-compensating package for fiber Bragg grating devices that protects the fiber optic device from handling, does not apply a mechanical torque to the fiber Bragg device, and allows the fiber Bragg grating to be written after installation in the package. An apparatus for temperature compensation of a fiber optic device, the apparatus comprises a housing member that has a longitudinal channel defined therein by first and second side walls. The first and second side walls each have a first height and a bottom surface. In addition, the housing has a first coefficient of thermal expansion. There is a longitudinal coupling region within the longitudinal channel in which the first and second side walls have a second height and the bottom surface has an aperture defined therethrough. The aperture allows optical communication between the coupling region and an external light source. The first and second thermal compensation members are sized and dimensioned to fit within the longitudinal channel, and are affixed within the longitudinal channel on opposite sides of the longitudinal coupling region. The first and second thermal compensation members each have a top surface that has a first length and a bottom surface that has a second length. The top surface includes a first region proximal to the coupling region, and a second region distal to the coupling region. The first region includes a plurality of spaced apart platforms, where each of the plurality of platforms is substantially the same height. The first and second thermal compensation members have a second coefficient of thermal expansion that is greater than the first coefficient of thermal expansion. A fiber optic device that includes a grating region that is disposed within the longitudinal coupling region, and is maintained at a predetermined level of tension. The grating region is maintained at this level of tension by attaching the fiber optic device being at opposite ends, where each end is affixed to one of the plurality of platforms in the first region of the first and second temperature compensation members respectively. Thus, as the temperature within the coupling region increases, the first and second temperature compensation members expand toward one another thereby reducing the level of tension within the grating region.
In another embodiment, an apparatus for temperature compensation of a fiber optic device, the apparatus comprises, a housing member having a longitudinal channel defined therein by first and second side walls. The first and second walls each have a first height and a bottom surface, and a first coefficient of thermal expansion. A longitudinal coupling region is disposed within the longitudinal channel, and within the coupling region the first and second side walls have a second height, and the bottom surface has an aperture defined therethrough. The aperture allows optical communication to exist between the coupling region and an external light source. The first and second thermal compensation members are sized and dimensioned to fit within the longitudinal channel, and are affixed within it on opposite sides of the longitudinal coupling region. The first thermal compensation member has a top surface having a first length and a bottom surface having a second length, and a first region proximal to the coupling region, and a second region distal to the coupling region. The first region includes a plurality of spaced apart platforms, and each of the plurality of platforms being substantially the same height. The first thermal compensation member has a second coefficient of thermal expansion that is greater than the first coefficient of thermal expansion. The second thermal compensation member has a top surface and a bottom surface. The top surface includes a first region proximal to the coupling region, and a second region distal to the coupling region. The first region includes a plurality of spaced apart platforms, and each of the plurality of platforms are substantially the same height as one another. The second thermal compensation member has a third coefficient of thermal expansion that is less than the second coefficient of thermal expansion. A fiber optic device containing a grating region that is disposed in the longitudinal coupling region, and the grating region is maintained at a predetermined level of tension. The grating region is maintained at this predetermined level of tension by attaching the fiber optic device at opposite ends, to one of the plurality of platforms in the first region of the first and second temperature compensation members respectively. Thus, as the temperature within the coupling region increases, the first temperature compensation members expands toward the second thermal compensation member thereby reducing the level of tension within the grating region.